Extraction of methanol from methyl tertiary butyl ether

Abstract: An impure methyl tertiary butyl ether product contaminated with isobutylene, methanol and water is purified by continuous counter-current contact with water and isobutylene in a counter-current contact extraction tower to provide an overhead raffinate comprising isobutylene, methyl tertiary butyl ether and water and an extract comprising methanol, water and a minor amount of methyl tertiary butyl ether, the overhead raffinate being separated in a methyl tertiary butyl ether purification distillation zone into a lighter distillation fraction comprising isobutylene and water and a heavier distillation fraction consisting essentially of methyl tertiary butyl ether, the lighter distillation fraction being decanted to remove water and to provide a distillate isobutylene fraction that is charged to a surge drum from which isobutylene is returned to the contact tower.

Description: This invention relates to the manufacture and isobutylene-assisted purification of methyl tertiary butyl ether. More particularly, this invention relates to a process for the manufacture of methyl tertiary butyl ether from tertiary butyl alcohol and methanol and to the purification of a methanol-contaminated methyl tertiary butyl ether intermediate product formed during the process; the intermediate product being purified by counter-current contact with water in an extraction tower, wherein isobutylene is added to the extraction tower to assist in the formation of a raffinate composed of methyl tertiary butyl ether, isobutylene and residual water and an extract comprising methanol, residual isobutylene, residual methyl tertiary butyl ether and water. Even more particularly, this invention relates to a method of purification of methanol-contaminated methyl tertiary butyl ether by counter-current contact with water in an extraction tower, wherein isobutylene is separately added to the extraction tower below the point at which the methanol-contaminated methyl tertiary butyl ether is introduced in order to selectively strip at least a portion of the residual methyl tertiary butyl ether from the extract. Methyl tert-butyl ether is finding increasing use as a blending component in high octane gasoline as the current gasoline additives based on lead and manganese are phased out. Currently all commercial processes for the manufacture of methyl tert-butyl ether are based upon the liquid-phase reaction of isobutylene and methanol catalyzed by a cationic ion-exchange resin. With the expanding use of MTBE as an acceptable gasoline additive, a growing problem is the availability of raw materials. Historically, the critical raw material is isobutylene (Oil and Gas J., June 8, 1987, p. 55). It would be advantageous, therefore, to have a process to make MTBE that does not require isobutylene as a building block. It would be advantageous to have an efficient process for making MTBE by reaction of methanol with tertiary butyl alcohol, since t-butanol (TBA) is readily available commercially through isobutane oxidation. It is known to react methanol with tertiary butyl alcohol in the presence of a catalyst in order to produce methyl tertiary butyl ether. A wide variety of catalysts have been suggested for this purpose. In US-A-2,282,469 there is disclosed a process for preparing methyl tertiary butyl ether over a catalyst comprising Kieselguhr impregnated with phosphoric acid at elevated temperature. The use of other catalysts is also known. Two of the principal by-products formed during the reaction of the methanol with the tertiary butyl alcohol are water and isobutylene. Methanol and methyl tertiary butyl ether form an azeotrope which is broken only with difficulty and therefore the separation of methanol from MTBE during the recovery of purified methyl tertiary butyl ether presents a serious problem. In US-A-4,820,877, separation of methanol from MTBE is accomplished by using a refinery fuel gas to enhance the separation of methanol into the overhead stream of a distillation column. In US-A-4,814,517, separation of methanol from MTBE is accomplished by using a silica gel to preferentially adsorb methanol from an MTBE stream and by periodically regenerating the silica gel. EP-A-0600632 (Texaco) describes a method for the preparation of methyl tertiary butyl ether, wherein a mixture of methanol and substantially peroxides-free tertiary butyl alcohol is catalytically reacted to form a reaction product that is separated into a first lighter distillation fraction comprising isobutylene, methanol and methyl tertiary butyl ether and a second heavier distillation fraction comprising methanol, tertiary butyl alcohol and water, wherein the first distillation fraction and a first recycle isobutylene fraction are reacted to form an isobutylene conversion product that is charged, together with recycle isobutylene to a methanol extraction zone and counter-currently contacted with water to provide an overhead extract comprising aqueous isobutylene, and wherein the isobutylene is recovered and recycled. In US-A-4,798,674, separation of methanol from MTBE is accomplished by using a membrane of cross-linked polyvinyl alcohol or a quaternary ammonium ion resin. Methanol preferentially permeates through the membrane increasing the MTBE concentration of the charge liquid. In US-A-4,759,850, separation of methanol from MTBE is accomplished by reverse osmosis. In US-A-4,440,963, separation of methanol from MTBE is accomplished by adding an agent such as 2-methyl pentane or Freon 113 to form an azeotrope with methanol. This azeotrope is recovered overhead giving a methanol-free MTBE bottoms product. As recognized by Rao et al. in US-A-4,144,138, isobutylene is formed as a by-product when methanol is reacted with tertiary butyl alcohol. In accordance with the Rao process, the isobutylene is separated from the reaction product in an initial azeotropic distillation step as a noncondensable gas. Rao taught that the isobutylene may be flashed from the reaction product for recycle, depending upon purity. In a process related to the present invention, by-product isobutylene is utilized downstream of the etherification reaction zone as a reactant, together with methanol, in the preparation of additional methyl tertiary butyl ether and in the purification of a methanol-contaminated methyl tertiary butyl ether intermediate product by counter-current contact with water in an extraction tower, isobutylene is used to facilitate in the removal of residual methyl tertiary butyl ether from the extract. In the continuous practice of the process of the present invention for the manufacture of methyl tertiary butyl ether and for the continuous purification of a methanol-contaminated methyl tertiary butyl ether intermediate product with water in an extraction tower, isobutylene is added to the extraction tower to assist in the purification. A comparatively large volume of isobutylene recycle is required for use in the extraction tower. However, process upsets can result in a temporary loss of produced methyl tertiary butyl ether and isobutylene. Such an event has the potential for shutting down the entire process, including the isobutylene-assisted water extraction of methanol from methyl tertiary butyl ether. In accordance with the present invention, an intermediate isobutylene storage drum, or surge drum, is provided upstream of the water extraction step to eliminate the potential for upsets in the water extraction step due to a shortage of isobutylene, and also to provide isobutylene inventory for start-ups and shut downs. Also, when tertiary butyl alcohol is prepared from tertiary butyl hydroperoxide, the tertiary butyl alcohol reaction product will contain minor amounts of peroxide contaminants such as tertiary butyl hydroperoxide, ditertiary butyl peroxide and allyl tertiary butyl peroxide. When tertiary butyl alcohol contaminated with minor amounts of peroxides is reacted with methanol to form methyl tertiary butyl ether, the etherification reaction product resulting from the reaction will contain the peroxide contaminants. In another process related to the present invention, a peroxides-contaminated tertiary butyl alcohol feedstock is passed through a peroxides decomposition reaction zone before being charged to the methyl tertiary butyl ether etherification reaction zone so that the methyl tertiary butyl ether etherification product will be substantially free from peroxide contaminants. In a further process related to the present invention, an integrated process is provided wherein a peroxides-contaminated tertiary butyl alcohol feedstock is passed through a peroxides decomposition reaction zone to substantially completely decompose the peroxide contaminants contained therein, and then charged to a methyl tertiary butyl ether etherification reaction zone together with methanol to form an isobutylene-containing methyl tertiary butyl ether etherification product that is substantially free from peroxide contaminants, wherein by-product isobutylene is utilized downstream of the etherification reaction zone as a stripping agent in the purification of the methyl tertiary butyl ether and wherein the by-product isobutylene is used as a reactant in the preparation of additional methyl tertiary butyl ether. In a process related to the present invention, a method for the continuous preparation of methyl tertiary butyl ether (MTBE) from tertiary butyl alcohol (TBA) and methanol (MeOH) is provided comprising the steps of: a) continuously charging a peroxides-contaminated tertiary butyl alcohol feedstock to a peroxides decomposition reaction zone and substantially completely decomposing the peroxide contaminants therein to form a substantially peroxides-free tertiary butyl alcohol product, b) continuously charging a reaction feed mixture comprising methanol and the substantially peroxides-free tertiary butyl alcohol product to an etherification reaction zone containing a bed of a TBA/MeOH etherification catalyst and reacting said reaction feed mixture therein to form an etherification reaction product comprising unreacted methanol, unreacted tertiary butyl alcohol, water, isobutylene (IBTE) and methyl tertiary butyl ether, c) continuously charging the etherification reaction product to a first methyl tertiary butyl ether distillation zone and separating it therein into a first lighter distillation fraction comprising isobutylene, methanol and methyl tertiary butyl ether and a second heavier distillation fraction comprising methanol, tertiary butyl alcohol and water, d) continuously charging an isobutylene reaction mixture comprising the first distillation fraction and a first recycle isobutylene (IBTE) fraction to an isobutylene conversion reaction zone containing a solid resin IBTE/MeOH etherification catalyst and partially reacting the isobutylene and methanol contained in the isobutylene reaction mixture to form an isobutylene conversion product, e) continuously charging the isobutylene conversion product to a methanol extraction zone comprising a counter-current extraction tower and counter-currently contacting the isobutylene conversion product therein with water to provide an overhead raffinate comprising isobutylene, methyl tertiary butyl ether and a minor amount of water and an extract comprising methanol, water and minor amounts of methyl tertiary butyl ether and isobutylene, f) continuously charging the raffinate to a second methyl tertiary butyl ether distillation zone and separating it therein into a third lighter distillation fraction comprising isobutylene and water and a fourth heavier distillation fraction consisting essentially of methyl tertiary butyl ether, g) continuously charging the third distillation fraction to a decantation separation zone and separating it therein into an isobutylene fraction and a water fraction, h) continuously recycling a first portion of said isobutylene fraction to the isobutylene conversion reaction zone as the first isobutylene recycle fraction, and i) continuously recycling a second portion of the isobutylene fraction to the methanol extraction zone as the methyl tertiary butyl ether stripping agent. Another process related to the present invention comprises a method for the continuous preparation of methyl tertiary butyl ether from tertiary butyl alcohol (TBA) and methanol (MeOH), including: b') continuously reacting a mixture of methanol and tertiary butyl alcohol in an etherification reaction zone containing a bed of a TBA/MeOH etherification catalyst to form an etherification reaction product comprising unreacted methanol, unreacted tertiary butyl alcohol, water, isobutylene (IBTE) and methyl tertiary butyl ether, c') continuously charging the etherification reaction product to a first methyl tertiary butyl ether distillation zone and separating it therein into a first lighter distillation fraction comprising isobutylene, methanol and methyl tertiary butyl ether and a second heavier distillation fraction comprising methanol, tertiary butyl alcohol and water, d') continuously charging an isobutylene reaction mixture comprising the first distillation fraction and a first recycle isobutylene (IBTE) fraction to an isobutylene conversion reaction zone containing a solid resin IBTE/MeOH etherification catalyst and partially reacting the isobutylene and methanol contained in the isobutylene reaction mixture to form an isobutylene conversion product, e') continuously charging the isobutylene conversion product to a methanol extraction zone comprising a counter-current contact tower and counter-currently contacting the isobutylene conversion product with water to provide an overhead raffinate comprising isobutylene, methyl tertiary butyl ether and a minor amount of water and an extract which is counter-currently stripped with isobutylene to remove residual amounts of methyl tertiary butyl ether. f') continuously charging the raffinate to a second methyl tertiary butyl ether distillation zone and separating it therein into a third lighter distillation fraction comprising isobutylene and water and a fourth heavier distillation fraction consisting essentially of methyl tertiary butyl ether, g') continuously charging the third distillation fraction to a decantation separation zone and separating it therein into an isobutylene fraction and a water fraction, h') continuously recycling a first portion of the isobutylene fraction to the isobutylene reaction zone as the first isobutylene recycle fraction, and i') continuously recycling a second portion of said isobutylene fraction to the methanol extraction zone as the methyl tertiary butyl ether stripping fraction. In a process related to the present invention, a method for the continuous preparation of methyl tertiary butyl ether is provided wherein the second heavier distillation fraction from the first methyl tertiary butyl ether distillation zone and the extract from the methanol extraction zone are further processed in order to recover the methyl tertiary butyl ether contained therein and in order to obtain the methanol and tertiary butyl alcohol contained therein for recycle, comprising the steps of: j) continuously charging the extract from the methanol extraction zone to a third methyl tertiary butyl ether distillation zone and separating it therein into a lighter fifth distillation fraction comprising methyl tertiary butyl ether and a heavier sixth distillation fraction comprising water and methanol, k) continuously charging the sixth distillation fraction to a fourth methanol distillation zone and separating it therein into a seventh lighter methanol recycle fraction and an eighth heavier distillation fraction, l) continuously charging the second heavier distillation fraction to a fifth distillation zone and separating it therein into a ninth lighter distillation recycle fraction comprising methanol and tertiary butyl alcohol, and a tenth heavier distillation fraction, m) continuously charging the fifth distillation fraction to the methanol extraction zone, and n) continuously charging the seventh distillation fraction and the ninth distillation fractions to the etherification reaction zone. The Etherification Reaction Catalyst An etherification reaction zone containing a bed of etherification catalyst is utilized. A wide variety of etherification catalysts can be used for this purpose, such as supported phosphorus acid-type catalysts. A preferred catalyst is a sulfonic acid resin etherification catalyst such as a sulfonated polystyrene resin crosslinked with divinyl benzene. Any suitable solid resin etherification catalyst may be used for this purpose, such as a strongly acidic ion exchange resin consisting essentially of sulfonated polystyrene, such as a divinyl benzene crosslink polystyrene matrix containing from about 0.5 to about 20% of copolymerized divinyl benzene. Resins of this nature are manufactured and sold commercially under various trade names such as "Dowex 50", "Nalcite HCR" and "Amberlyst 15". The use of catalyst of this nature is disclosed, for example, in Rao US-A-4,144,138. Also, Kieselguhr impregnated with phosphoric acid as disclosed in Frolich US-A-2,282,469, titania having phosphoric acid impregnated thereon as disclosed in Knifton US-A-4,822,921, a hetero polyacid such as 12-tungstophosphoric acid or 12-molybdophosphoric acid supported on titania may be used. Zeolites as disclosed in JP-B-0007432 or aluminosilicate zeolites as disclosed in Chang et al. US-A-4,058,576 may also be used. The reaction conditions to be utilized when reacting methanol with tertiary butyl alcohol in the presence of a sulfonic acid resin etherification catalyst of the type disclosed in the prior art include a reaction temperature of 90° to 140°C, a pressure of 0.2 to 3.4 MPa (30 to 500 psia) and a space velocity of 0.5 to 20 volumes of feed per volume of etherification catalyst per hour. The Solid Resin Etherification Catalyst A distillate fraction obtained during the recovery process, identified above as the first distillation fraction, and which contains both isobutylene and methanol is brought into contact with a solid resin etherification catalyst, as described above, in order to convert a significant portion of the isobutylene and methanol to methyl tertiary butyl ether. The first distillation fraction will normally contain from 5 to 10 wt.% of isobutylene, from 70 to 80 wt.% of methyl tertiary butyl ether and from 10 to 20 wt.% of methanol. An isobutylene reaction mixture is prepared from the first distillation fraction and a first isobutylene recycle fraction so as to provide an isobutylene reaction mixture containing about an additional 1 to 10 wt.% of isobutylene, based on the weight of the first distillation fraction. Thus, the isobutylene reaction mixture may comprise from 6 to 15 wt.% of isobutylene, from 65 to 75 wt.% of methyl tertiary butyl ether and from 10 to 20 wt.% of methanol. Any suitable solid resin etherification catalyst may be used for this purpose, such as a strongly acidic ion exchange resin consisting essentially of sulfonated polystyrene, such as a divinyl benzene crosslink polystyrene matrix containing from 0.5 to 20% of copolymerized divinyl benzene. Resins of this nature are manufactured and sold commercially under various trade names such as "Dowex 50", "Nalcite HCR" and "Amberlyst 15". It is contacted with a solid resin etherification catalyst in the isobutylene conversion reaction zone under conversion conditions including, for example, a temperature of 35° to 130°C, a pressure of 0.2 to 3.4 MPa (30 to 500 psia) and a contact time of 0.5 to 20 volumes of first distillate fraction per volume of etherification catalyst per hour. As a consequence, an isobutylene conversion product is formed which will normally contain from 0 to 10 wt.% of isobutylene, 75 to 85 wt.% of methyl tertiary butyl ether and from 10 to 15 wt.% of methanol. Peroxide Decomposition When the tertiary butyl alcohol feedstock to be used in the preparation of methyl tertiary butyl ether is tertiary butyl alcohol contaminated with peroxides such as tertiary butyl hydroperoxide, ditertiary butyl peroxide, allyl tertiary butyl peroxide, the feedstock is treated for the substantially complete removal of the peroxide contaminants before it is charged to the methyl tertiary butyl ether etherification zone. It is known to prepare tertiary butyl alcohol by the thermal or catalytic decomposition of tertiary butyl hydroperoxide. It is also known to prepare tertiary butyl alcohol by the catalytic reaction of tertiary butyl hydroperoxide with propylene to form propylene oxide and tertiary butyl alcohol. The tertiary butyl alcohol feedstock derived from tertiary butyl hydroperoxide in this manner will contain peroxide contaminants. A typical feedstock prepared in this fashion will contain from 95 to 99 wt.% of tertiary butyl alcohol and less than 2.0 wt.% of peroxide contaminants. The peroxides-contaminated tertiary butyl alcohol is charged to a peroxides decomposition reaction zone where the peroxides are substantially completely thermally and/or catalytically decomposed. The peroxide contaminants will be decomposed to form water and tertiary butyl alcohol, and trace amounts of other decomposition products such as acetone and methyl formate. When the peroxides are to be thermally decomposed, the peroxides-contaminated tertiary butyl alcohol feedstock is continuously passed through a reactor in the peroxides decomposition reaction zone at a temperature of 100° to 200°C, a pressure of 0.6 to 3.4 MPa (80 to 500 psia) at a flow rate of 0.5 to 20 volumes of feedstock per reactor volume per hour to thereby provide a substantially peroxides-free tertiary butyl alcohol reaction product. Alternately, the peroxide contaminants may be catalytically decomposed. A wide variety of catalysts may be used for this purpose, such as cobalt borate, a nickel, copper, chromia catalyst, an iron, copper, chromia, cobalt catalyst, a base treated hydrogenation catalyst from groups VIB or VIIIB of the Periodic Table, a nickel, copper, chromium and barium catalyst, a metal phthalocyanine catalyst, an imidazole-promoted methyl metal phthalocyanine catalyst, a base promoted metal phthalocyanine catalyst, a solid ruthenium catalyst or a promoted metal porphine catalyst. The conversion conditions to be utilized in the peroxide decomposition zone may comprise, for example, a temperature of 100° to 200°C, a pressure of 0.6 to 3.4 MPa (80 to 500 psia) and a space velocity of 0.5 to 20 volumes of feed per volume of catalyst per hour. The effluent from the peroxide decomposition zone will typically comprise 95 to 99 wt.% of tertiary butyl alcohol and less than 0.1 wt.% of peroxide contaminants. The invention will now be described with reference to the accompanying Drawings, in which: Fig. 1 is a schematic flow sheet with conventional parts omitted showing the general reaction and recovery sequence for the manufacture and purification of methyl tertiary butyl ether; and Fig. 2 is a fragmentary schematic flow sheet, with conventional parts omitted, showing part of the general sequence of Fig. 1 and illustrating the method according to the present invention for the purification of a methanol-contaminated methyl tertiary butyl ether intermediate product by counter-current contact with water in an extraction tower, using isobutylene to strip most of the residual methyl tertiary butyl ether from the extract. Referring to Fig. 1, there is provided an etherification reaction zone 10 containing a bed of solid etherification catalyst. Any suitable etherification catalyst may be used such as, for example, a solid resin etherification catalyst of the type described above, such as a strongly acidic ion exchange resin consisting essentially of sulfonated polystyrene crosslinked with divinyl benzene (e.g., Dowex 50, Nalcite HCR, Amberlyst 15). As another example, the catalyst may be a fluorophosphoric acid-on-titania catalyst or a heteropoly acid such as 12-tungstophosphoric acid or 12-molybdophosphoric acid supported on an inert support such as titania. When the tertiary butyl alcohol is prepared by the thermal or catalytic decomposition of tertiary butyl hydroperoxide, it will contain minor amounts of impurities such that, for example, the feedstock charged to the reaction zone 10 will contain the following components: ETHERIFICATION REACTION ZONE FEED MIXTURE Component wt.% (approximate) Methanol 41.0 TBA 47.0 Acetone 0.5 2-Propanol 6.0 MTBE 0.2 DTBP 0.1 t-Butyl Formate 0.1 Water 6.0 100.9% When the tertiary butyl alcohol feedstock to be initially charged to etherification reaction zone 10 is a peroxides-contaminated tertiary butyl alcohol feedstock, as described above, the tertiary butyl alcohol feedstock is initially charged by way of a tertiary butyl alcohol feed line 13 to a peroxides decomposition zone 11, such as, for example a thermal peroxides decomposition zone 11 operated, (e.g.) at a temperature of 100° to 200°C, a pressure of 0.6 to 3.4 MPa (80 to 500 psia) and a flow rate of 0.5 to 20 volumes of feedstock per reactor volume per hour to thereby provide a substantially peroxides-free tertiary butyl alcohol reaction product. The peroxide contaminants will be decomposed to form water and tertiary butyl alcohol, and trace amounts of other decomposition products such as acetone and methyl formate. The substantially peroxides-free tertiary butyl alcohol reaction product is continuously discharged from the peroxides decomposition zone 11 by a discharge line 17 leading to a manifold 12. Fresh methanol is continuously charged to the manifold 12 by a line 15, as is a recycle stream 112 containing recycle methanol and recycle tertiary butyl alcohol. The flow of methanol and tertiary butyl alcohol to the manifold 12 through the lines 15, 17 and 112 is regulated so that a molar excess of methanol is present in the line 14 leading to the etherification reaction zone 10, such as, for example, a molar ratio of 1.1 to 3 moles of methanol per mol of tertiary butyl alcohol. Within the etherification reaction zone 10, the feed mixture is brought into contact with a bed of etherification catalyst, such as a sulfonic acid resin etherification catalyst under reaction conditions including a pressure of 0.2 to 3.4 MPa (30 to 500 psia), and more preferably from 1.4 to 2.0 MPa (200 to 300 psia), a temperature of 30° to 200°C, and more preferably from 80° to 140°C, and still more preferably from 90° to 130°C. When the catalyst is a supported phosphorus acid-type catalyst, the reaction temperature may suitably be in the range of 150° to 190°C. Contact time within the etherification reaction zone is suitably such that 0.5 to 20 volumes of feed mixture per volume of etherification catalyst per hour are fed to the etherification reaction zone 10 and, more preferably from 1 to 4 volumes of feed mixture per volume of etherification catalyst per hour. Within the etherification reaction zone 10, methanol will exothermically react with the tertiary butyl alcohol to form methyl tertiary butyl ether which will be contained in a reaction product discharged from the etherification reaction zone 10 by way of a line 20 leading to a first methyl tertiary butyl ether (MTBE) distillation zone 30. As a specific example, when the solid etherification catalyst is a sulfonic acid resin such as Amberlyst 15 and when the molar ratio of methanol to tertiary butyl alcohol in the feed mixture charged to the etherification reaction zone 10 by the line 14 is within the ratio of 2.0 moles of methanol per mole of tertiary butyl alcohol, and the reaction is conducted at a temperature of 110°C at a feed rate of 2.0 volumes of feed mixture per volume of catalyst per hour, the etherification reaction product may have the composition in part shown by the following table: ETHERIFICATION REACTION PRODUCT Component wt.% (Approx.) Water 14.0 Methanol 27.6 Isobutylene 3.0 TBA 14.1 MTBE 34.5 Other 6.8 The etherification reaction product charged to the first MTBE distillation zone 30 by way of the charge line 20 is fractionated therein under distillation conditions including a liquid reflux temperature of 30° to 100°C, and more preferably 40° to 80°C, a reboiler temperature of 80° to 115°C, and more preferably from 95° to 105°C, and a pressure of 0.1 to 0.4 MPa (15 to 60 psia), the distillation condition being selected such that substantially all of the MTBE in the etherification reaction product 20 is taken overhead from the first distillation zone 30 by a line 32. As a consequence, the first distillation fraction 32 taken overhead from the distillation zone 30 will comprise substantially all of the isobutylene and substantially all of the methyl tertiary butyl ether and some of the methanol charged to the first distillation zone 30. The second heavier distillation fraction 34 discharged from the first MTBE distillation zone 30 will comprise methanol, tertiary butyl alcohol and water. The first distillation fraction 32 in admixture with recycle isobutylene added by a recycle line 89 is charged by way of manifold 33 and charge line 35 to an isobutylene conversion zone 40 containing a bed of solid resin etherification catalyst such as a bed of Amberlyst 15 sulfonated polystyrene-divinyl benzene copolymer acidic ion exchange resin. Suitably, the admixture in the line 35 will contain 2 to 15 parts of recycle isobutylene per 100 parts of first distillation fraction 32. Etherification reaction conditions established in the isobutylene conversion zone 40 include, for example, a temperature of 35° to 130°C, and more preferably from 40° to 70°C, a pressure of 0.3 to 3.4 MPa (50 to 500 psia), and more preferably from 1.0 to 1.7 MPa (150 to 250 psia), and a contact time of 0.5 to 4 volumes of first distillation fraction per volume of solid resin etherification catalyst per hour. As a consequence, a portion of the methanol and isobutylene contained in the first distillation fraction 32 will be converted to methyl tertiary butyl ether. Typically, the conversion will amount to 30 to 60 wt.%, based on the isobutylene. As a consequence, there will be formed an isobutylene conversion product discharged from the isobutylene conversion zone 40 by a line 42 leading to a methanol solvent extraction zone 50. The composition of a typical isobutylene conversion product may be characterized as follows: ISOBUTYLENE CONVERSION PRODUCT Component wt.% (Approx.) Isobutylene 5.4 MTBE 79.5 Methanol 12.2 Other 2.9 The isobutylene conversion product 42 together with recycle isobutylene charged by a recycle line 83 is charged through line 41 to a manifold 44 and thence via line 46 to a methanol solvent extraction zone 50 where it is counter-currently contacted with water introduced into the solvent extraction zone 50 by a charge line 52 controlled by a valve 54. Within the methanol solvent extraction zone 50, solvent extraction conditions are established for countercurrent solvent extraction including a ratio of water to extraction feed mixture within the range of 0.05 to 0.3 parts of water per part of extraction feed mixture per hour, and more preferably a ratio of 0.08 to 0.2 parts of water per part of extraction feed mixture. Extractive conditions to be established may suitably include a temperature of 20° to 60°C, and more preferably from 30° to 40°C, and a pressure of 0.3 MPa to 3.4 MPa (50 to 500 psia), and more preferably from 0.3 to 1.0 MPa (50 to 150 psia). As a consequence, a supernatant raffinate will be formed which is withdrawn from the methanol solvent extraction zone 50 by line 60. The extract is discharged from the solvent extraction zone 50 by way of a bottoms charge line 64 leading to a third methyl tertiary butyl ether distillation zone 70. Within the second methyl tertiary butyl ether purification distillation zone 62, distillation conditions are established including a liquid reflux temperature of 30° to 60°C, and more preferably from 40° to 55°C, a reboiler temperature of 100° to 140°C, and more preferably from 125° to 135°C and a pressure of 0.5 to 0.8 MPa (70 to 120 psia), and more preferably from 0.6 to 0.75 MPa (90 to 110 psia), to thereby form a lighter distillation fraction 66 discharged from the second distillation zone 62 and a heavier fourth distillation fraction 68 consisting essentially of product, namely methyl tertiary butyl ether. The third distillation fraction 66 will comprise a mixture of isobutylene and water and suitably charged to a decantation zone 80 where it can settle to form a supernatant isobutylene phase withdrawn from the decantation zone 80 by way of a line 82. Water is discharged from the decantation zone 80 by way of a water discharge line 84 and is suitably purged from the system. A portion of the isobutylene in the line 82 is recycled by way of manifold 86, line 89 controlled by valve 87, manifold 33 and line 34 to the isobutylene conversion reaction zone 40 as a first isobutylene recycle fraction and another portion of the isobutylene in the line 82 is recycled to the methanol solvent extraction zone 50 through a branch line 83 controlled by a valve 85 as a second isobutylene recycle fraction. Suitably, 10 to 15 wt.% of the isobutylene in line 82 is discharged from manifold 86 as the first isobutylene recycle fraction and 90 to 85 wt.% is discharged as the second isobutylene recycle fraction. The extract 64 charged to the third distillation zone 70 will comprise methanol, water and residual quantities of methyl tertiary butyl ether and isobutylene, and is suitably fractionated therein under distillation conditions including a liquid reflux temperature of 30° to 90°C, and more preferably from 50° to 75°C, and a reboiler temperature of 80° to 120°C, and more preferably from 105° to 115°C, and a pressure of 0.1 to 0.4 MPa (15 to 60 psia), and more preferably from 0.25 to 0.3 MPa (40 to 50 psia), to form a fifth lighter distillation fraction 72 comprising methyl tertiary butyl ether which may suitably be charged to the manifold 44 for the charge line 46 to the methanol solvent extraction zone 50. A sixth heavier distillation fraction comprising water and methanol is discharged from the third distillation zone 70 by a line 74 leading to a fourth distillation zone 90. The sixth distillation fraction charged to the fourth methanol distillation zone 90 is fractionated therein under distillation conditions which may suitably include a liquid reflux temperature of 30° to 80°C, and more preferably from 60° to 75°C, a reboiler temperature of 100° to 140°C, and more preferably from 110° to 120°C, and a pressure of 0.1 to 0.4 MPa (15 to 60 psia), and more preferably from 0.15 to 0.2 MPa (20 to 30 psia), into a seventh lighter distillation fraction 92 which may be suitably charged to a manifold 93 and from thence by line 112 to the manifold 12 for recycle to the etherification reaction zone 10 through feed line 14. A heavier distillation fraction consisting essentially of water is discharged from the fourth methanol distillation zone by way of a line 94 and may be discharged from the system through manifold 95 and water discharge line 96. The second distillation fraction 34 discharged from the first MTBE distillation zone 30 in accordance with the present invention is charged to a fifth tertiary butyl alcohol recovery distillation zone 100 where it is fractionated under distillation conditions including a liquid reflux temperature of 35° to 170°C, and more preferably 140° to 150°C, and a reboiler temperature of 100° to 190°C, more preferably 170° to 180°C, and at a pressure of 0.1 to 1.3 MPa (15 to 190 psia), and more preferably 0.75 to 1.1 MPa (110 to 160 psia), into a ninth distillation fraction discharged from the fifth distillation zone 100 by a line 102 leading to manifold 103 and a tenth heavier distillation fraction comprising a third water fraction discharged from the distillation zone 100 by a line 106 leading by way of manifold 95 to the water discharge line 96. The ninth distillation fraction 102 is charged to a manifold 103 provided with a discharge line 118 controlled by a valve 116 leading to the feed manifold 12 so that the methanol can be directly charged to the etherification reaction zone 10 from manifold 12 through feed line 14. A second branch line 120 controlled by a valve 98 is provided which leads from manifold 103 to the feed manifold 44 for the methanol extraction zone 50 so that all or a portion of the ninth distillation fraction 102 may be recycled to the manifold 44. Turning now to Fig. 2, it will be noted that the isobutylene discharged from the decantation zone 80 by the line 82 is charged to a manifold 200 from which a portion of the isobutylene is charged to the recycle line 89 for processing as described above. Another portion of the isobutylene is discharged from the manifold 200 by a branch line 202 controlled by a valve 204 leading to a surge tank 210, which will suitably be of a capacity adequate to hold an inventory of isobutylene sufficient to provide for 12 to 36 hours of continuous operation of the water extraction zone 50. In accordance with the present invention, the water extraction zone 50 is suitably a counter-current extraction tower of the type known to those skilled in the art. However, it is operated in a manner different from the manner described above. Isobutylene is discharged from the surge drum 210 by a line 212 controlled by a valve 214 leading to a charge point below the point at which the isobutylene conversion product 46 is charged to the water extraction zone 50; being preferably charged 1 to 3 theoretical plates below the point of introduction of the isobutylene conversion product 46 and 0 to 1 theoretical plates above the bottom of the water extraction zone 50 so as to selectively contact only the extract. Extraction conditions established in the extraction tower may suitably include a temperature of 20° to 60°C, and more preferably 30° to 40°C, a pressure of 0.35 to 3.4 MPa (50 to 500 psia) and more preferably 0.35 to 1.03 MPa (50 to 150 psia), and a residence time of 4 hours to 8 hours for the aqueous phase and from 0.04 hour to 0.10 hour for the organic phase. The water suitably may be charged by the line 52 to the extraction zone 50 in the ratio of 0.05 to 0.3 part of water per part of extraction feed charged to the extraction zone 50 by the line 46, and more preferably in the ratio of 0.08 to 0.2 part of water per part of extraction feed charged to the extraction zone 50. Isobutylene suitably may be charged by the line 212 to the extraction zone 50 in the ratio of 0.3 to 1 part of isobutylene per part of methyl tertiary butyl ether in the extraction feed charged to the extraction zone 50 by the line 46, and more preferably in the ratio of 0.5 to 0.7 part of isobutylene per part of methyl tertiary butyl ether in the extraction feed charged to the extraction zone 50 by the line 46. The raffinate 60 discharged from the methanol extraction zone 50 is charged to the second MTBE distillation zone 62, which is operated in the manner described above in order to provide a lighter fraction 66 comprising a mixture of water and isobutylene and a heavier MTBE product fraction 68. The lighter fraction 66 is charged to the decantation zone 80 wherein isobutylene is decanted and discharged by the line 82 for processing as described above and wherein the water is accumulated for discharge by the line 84. The extract fraction 64 discharged from the methanol extraction zone 50 by the line 64 is processed in the manner described above. In a process related to the present invention, a tertiary butyl alcohol feedstock is continuously charged to peroxide decomposition zone 11 by a line 13 where it is thermally treated under thermal peroxide decomposition conditions including a temperature of 100° to 200°C, a pressure of 0.6 to 3.4 MPa (80 to 500 psia) and a flow rate of 0.5 to 4 volumes of feedstock per reactor volume per hour to thereby provide a substantially peroxides-free tertiary butyl alcohol reaction product. The peroxides-contaminated feedstock and the substantially peroxides-free reaction product discharged from the peroxide decomposition zone 11 will typically have compositions as follows: PEROXIDE DECOMPOSITION ZONE FEED AND PRODUCT Component Approx. Feed (wt.%) Approx. Product (wt.%) ATBP 0.39 0.00 DTBP 0.87 0.02 TBA 97.2 97.4 Water 0.1 0.02 Other 1.44 2.56 The substantially peroxides-free tertiary butyl alcohol reaction product 17 is charged to the manifold 12 together with fresh methanol feedstock 15 and the recycle fraction 112 in amounts, for example, such that the charge ratio of methanol to tertiary butyl alcohol in the feed line 14 amounts to 2 moles of methanol per mole of tertiary butyl alcohol. The feed mixture is discharged from the manifold 12 by a line 14 leading to etherification reaction zone 10 containing a bed of a suitable etherification catalyst, such as Amberlyst 15 catalyst. Within the etherification reaction zone 10, the feedstock is passed through the etherification reaction bed on a continuous basis under reaction conditions, as described above, to thereby provide a reaction product having the following composition: ETHERIFICATION REACTION ZONE 10 REACTION PRODUCT Component wt.% (Approx.) Methanol 27.6 TBA 14.5 Water 14.0 Isobutylene 3.0 MTBE 34.5 Acetone 0.4 2-Propanol 6.0 The etherification zone reaction product is discharged from the reaction zone 10 by a line 20 leading to first methyl tertiary butyl ether distillation zone 30 where the fraction 20 is separated into a first lighter distillation fraction 32 comprising 6.5 wt.% isobutylene, 16.5 wt.% methanol, 75 wt.% MTBE and 2 wt.% other components, and a second heavier fraction comprising 37 wt.% methanol, 26.0 wt.% tertiary butyl alcohol, 25.5 wt.% water, 11 wt.% isopropanol and 0.5 wt.% of other components. The first distillation fraction 32 along with recycle isobutylene 89 is continuously charged to an isobutylene conversion zone 40 through the line 34 and brought into contact therein with a solid resin etherification catalyst, such as Amberlyst 15 catalyst, under conversion conditions, as described above, to thereby convert 50 wt.% of the isobutylene and a portion of the methanol in the first distillation fraction to MTBE and to form an isobutylene conversion product which is discharged from the isobutylene reaction zone 40 by a line 42 and which typically has the following composition: ISOBUTYLENE CONVERSION FEED AND PRODUCT, wt.% Component Approx. wt.% Feed Approx. wt.% Product Isobutylene 11 5.5 MTBE 71 79.5 Methanol 15 12 Other 3 3 The isobutylene conversion fraction 42 and the isobutylene recycle fraction 83 are continuously charged to the manifold 44 by a line 41 leading from manifold 45. The isobutylene mixture is discharged from the manifold 44 together with recycle fraction 120 and recycle fraction 72 by a feed line 46 leading to methanol extraction zone 50 in the ratio of 2 parts of isobutylene conversion product per part of recycle isobutylene. Water is charged to the methanol extraction zone 50 by a water charge line 52 in an amount such that the ratio of water to isobutylene and isobutylene conversion product in the methanol extraction zone 50 is in the range of 0.05 to 0.3 parts of water per part of extraction zone feed mixture. Within the methanol extraction zone, the methanol is extracted from the isobutylene conversion product under extraction conditions as described above to thereby provide an overhead raffinate fraction 60 comprising isobutylene and methyl tertiary butyl ether and residual quantities of water and an extract 64 comprising methanol, water and residual quantities of isobutylene and methyl tertiary butyl ether. The raffinate is fed by a line 60 to a second methyl tertiary butyl ether purification distillation zone 62 where it is resolved by distillation into a third lighter distillation fraction 66 comprising isobutylene and water and into a fourth heavier distillation fraction 68 consisting essentially of methyl tertiary butyl ether which is discharged as product. The third distillation fraction 66 is charged to a decantation separation zone 80 where it is permitted to settle and is resolved into an isobutylene fraction 82 and a first water fraction 84 which is discharged from the system. The isobutylene fraction 82 is suitably recycled to the isobutylene conversion zone 40 in admixture with the first distillation fraction 32 and to the feed to the methanol extraction zone 50. Typically, 2 to 10 parts of recycle isobutylene from the line 89 will be mixed with 100 parts of overhead product from the first distillation fraction 32. The extract 64 is continuously charged to a third methyl tertiary butyl ether distillation zone 70 where it is separated into a lighter fifth distillation fraction 72 comprising methyl tertiary butyl ether and a sixth heavier distillation fraction 74 comprising methanol and water. The fraction 74 is continuously charged to a fourth methanol distillation zone 90 wherein it is separated by fractional distillation into a seventh lighter distillation fraction 92 comprising methanol and an eighth distillation fraction comprising water which is discharged by a line 94. In accordance with the preferred embodiment of the present invention, the fraction 92 together with fraction 118 is charged by a line 112 to feed manifold 12. The second distillation fraction 34 from the first MTBE distillation zone 30 is continuously charged to a fifth tertiary butyl alcohol recovery distillation zone 100 where it is separated into a ninth lighter distillation fraction 102 comprising methanol, tertiary butyl alcohol and water and a tenth heavier distillation fraction 106 comprising water. The tenth heavier distillation fraction is discharged by a line 106 and the water in line 106, together with the water in line 94, may suitably discharge from the system from manifold 95 through water discharge line 96. EXAMPLES Example 1 As a comparative example of the present invention, as illustrated in Fig. 1, the extraction column 50 consisting of 7 theoretical stages may be operated at a temperature of 37°C and a pressure of 1.1 MPa (155 psia). An extraction feedstock comprising a mixture of the isobutylene conversion product and recycled isobutylene is charged to the bottom stage of extraction column 50 by the line 46 and water is charged to the top stage by line 52 in the ratio of 11 parts of extraction feedstock per part of water. The composition of the extraction feedstock, the charge water, the raffinate and the extract is given in the following table. Note from Table 1 the substantial absence of methanol in the raffinate and the substantial presence of MTBE in the extract. Solvent Extraction of Methanol from Methanol-contaminated Methyl Tertiary Butyl Ether Item Extraction Feed Water Extract Raffinate wt.% Parts wt.% Parts wt.% Parts wt.% Parts Methanol 8.93 89 0.80 1 41.59 86 0.49 4 MTBE 55.82 558 0 0 11.75 24 60.42 534 Water 0.10 1 99.20 99 41.00 84 0.64 6 IBTE 31.60 316 0 0 2.63 5 35.14 311 Other 3.55 36 0 0 3.03 7 3.31 29 Total (Parts) 1000 100 206 884 Solvent Extraction of Methanol from Methanol-Contaminated Methyl Tertiary Butyl Ether Item Extraction Feedstock Water Extract Raffinate Recycle Isobutylene wt.% Parts wt.% Parts wt.% Parts wt.% Parts wt.% Parts Methanol 12.37 124 0.80 1 45.74 122 0.53 7 1.01 4 MTBE 79.38 794 0 0 0.33 1 61.71 800 1.54 7 Water 0.07 1 99.20 128 45.17 121 0.66 8 0.16 1 IBTE 5.61 56 0 0 4.51 12 34.04 441 91.48 397 Other 2.57 25 0 0 4.25 11 3.06 40 5.81 25 Total (Parts) 1000 129 267 1296 434 Example 2 As an example of the present invention, as illustrated in Fig. 2, the extraction column 50 consisting of 7 theoretical stages may again be operated at a temperature of 37°C and a pressure of 1.1 MPa (155 psia). An extraction feedstock comprising the isobutylene conversion product is charged to the extraction column 50 by the line 46, recycle isobutylene is charged by the line 212 and water is charged by line 54 to the top stage in the ratio of 11 parts of extraction feedstock plus recycle isobutylene per part of water. The recycle isobutylene is fed 2 theoretical stages below the extraction feedstock. The composition of the extraction feedstock, the charge water, the raffinate, the extract and the recycle isobutylene is given in the following table. Note from Table 2 not only the substantial absence of methanol in the raffinate but also the almost total absence of methyl tertiary butyl ether from the extract. Example 3 (Comparative) Description of Pilot Plant Methanol Extraction Column As an additional example of the present invention, as illustrated in Figure 1, a pilot unit was operated for 6 days. The MTBE pilot plant methanol extraction column consists of three, 152.4 cm (5') flanged sections of 3.8 cm (1.5"), 316 stainless steel, schedule 40 pipe. Each of the three sections is packed with 6 mm 1/4" ceramic raschig rings providing a total of 475.4 cm (15') of packing. Temperature control is maintained through electrical resistance heating coils wrapped around the outer surface of the column and extending over the entire length of the column. Feed temperature control is maintained through electrical resistance heaters on both the aqueous and organic feed lines. The column pressure is controlled through a pressure control valve on the raffinate stream. The level of the interface between the organic and aqueous layers in the column is indicated through a differential pressure transmitter and controlled by a level control valve on the extract line. The interface level can also be viewed in a sight glass attached to the column. The organic and aqueous flows to the column are monitored through Coriolis type flow meters and are controlled through flow control valves. Overall material balance is verified by digital scales on the feed, raffinate, and extract tanks. The column and all supporting equipment are connected to a digital control system which provides both data gathering and control. Results from Pilot Plant Tests Feed Water Raffinate Extract Methanol wt.% (parts) 9.08 (91) 0.00 (0) 0.18 (2) 34.50 (87) MTBE wt.% (parts) 56.55 (566) 0.00 (0) 60.18 (548) 10.38 (26) Water wt.% (parts) 0.24 (2) 100.00 (137) 1.84 (17) 50.40 (127) IBTE wt.% (parts) 33.37 (334) 0.00 (0) 37.23 (339) 4.13 (10) Other wt.% (parts) 0.76 (7) 0.00 (0) 0.57 (5) 0.59 (2) Total Fl<-> (parts) ow 1000 137 911 252

Claims: A method for the continuous purification of methyl tertiary butyl ether contaminated with isobutylene, methanol and water, which comprises: continuously charging an extraction feed comprising water, isobutylene, methanol and methyl tertiary butyl ether to a methanol extraction zone comprising a counter-current contact tower and countercurrently contacting said extraction feed therein with water to provide an organic phase overhead raffinate comprising isobutylene, methyl tertiary butyl ether and a minor amount of water and a water phase extract comprising methanol, water and a minor amount of methyl tertiary butyl ether and stripping said extract with an isobutylene fraction in said extraction tower to remove methyl tertiary butyl ether therefrom, continuously charging said raffinate to a methyl tertiary butyl ether purification distillation zone and separating said raffinate therein into a lighter distillation fraction comprising isobutylene and water and a heavier distillation fraction consisting essentially of methyl tertiary butyl ether; continuously charging said lighter distillation fraction to a decantation separation zone and separating it therein into a distillate isobutylene fraction and a water fraction; continuously accumulating isobutylene from said distillate isobutylene fraction in a surge drum, continuously returning distillate isobutylene from said surge drum to said counter-current contact tower at a charge point 1 to 3 theoretical plates below the point of introduction of said extraction feed. A method as claimed in claim 1 wherein the extraction conditions in the extraction tower include a temperature of 20° to 60°C., a pressure of 0.35 to 3.4 MPa (50 to 500 psia) and a residence time of 4 to 8 hours for the water phase and from 0.04 to 0.10 hour for the organic phase. A method as claimed in claim 1 or claim 2 wherein a ratio of 0.05 to 0.3 parts of water per part of extraction feed and a ratio of 0.3 to 1 part of isobutylene per part of methyl tertiary butyl ether in the extraction feed are maintained in the extraction tower. A method as claimed in claim 1 wherein the extraction conditions in the extraction tower include a temperature within the range of 30° to 40°C., a pressure of 0.3 to 1.0 MPa (50 to 150 psia) and a residence time of 5 to 7 hours for the water phase and from 0.06 to 0.08 hour for the organic phase. A method as claimed in claim 4 wherein a ratio of 0.08 to 0.2 parts of water per part of extraction feed and a ratio of 0.5 to 0.7 part of isobutylene per part of methyl tertiary butyl ether in the extraction feed are maintained in the extraction tower. A method as claimed in anyone of claims 1 to 5 wherein the surge drum has a capacity adequate to provide for 12 to 36 hours of continuous operation of said counter-current contact tower.

Inventors: CASSATA JOHN ROBERT, CONKWRIGHT CATHERINE JUNE, HWAN REI-YU JUDY, PARKER STUART JOHN, CASSATA, JOHN ROBERT, CONKWRIGHT, CATHERINE JUNE, HWAN, REI-YU JUDY, PARKER, STUART JOHN
Applicants: HUNTSMAN SPEC CHEM CORP, HUNTSMAN SPECIALTY CHEMICALS CORPORATION
Application year: 1994
Main classification: C07C43
IPC codes: C07C41, C07C43